Eddy-current testers have been in use for many years for testing manufactured rod or wire. For instance, the Institut Dr. Forster of West Germany makes several models of eddy-current testers presently in use. Such testers typically provide, at an output thereof, a fault signal whenever a disturbance in eddy-currents within a test piece indicates a flaw or defect in the rod or wire. The fault signals may typically be of two types, i.e., major and minor. These signals are sometimes provided to a CRT display (see, for example, FIG. 3 of U.S. Pat. No. 3,904,957) for continuous visual monitoring, to a defect counter for providing a total defect count (see Column 1, line 40 of U.S. Pat. No. 3,904,957), or to a stripchart recorder upon which a pen inscribes a horizontal line which provides a spike in the presence of a fault. The defect counter is useful for viewing, from time to time, the total number of faults which have occurred so far in a workpiece and hence, its overall quality. However, the defect counter is not useful for indicating the rate of increase of faults unless viewed more or less continually. Knowledge of the rate of increase is useful for other purposes, to be described subsequently herein.
In the stripchart recorder approach, the stripchart is rolled up on a cylinder for later viewing by unrolling the paper periodically for evaluation. Two pens may be provided for providing two horizontal lines, one for major, and the other for minor faults. This method of recording data generates a considerable amount of paper. The operator must manually sort through and count all the faults in order to come to any useable conclusions concerning the quality of the product. If the operator should desire to determine the rate of increase of fault signals he is faced with the twin difficulties of having to unroll the unwieldy paper and at the same time attempt to qualitatively determine the rate of increase by inspection. A quantitative measurement is not available except by painstaking manual measurement and calculation. Furthermore, the equipment provided is typically not only very expensive but it requires considerable expenditure of man-hours to correlate data.